This invention relates to a control system for a DC motor, particularly to a system having a DC-DC switching converter.
In a conventional system, when controlling the rotational speed of a DC motor, a drive transistor is used to supply the DC motor with a controlled voltage from a DC voltage source. The drive transistor controls the electrical power to the DC motor by changing the voltage drop between the collector and emitter terminals. In this case, the supplied power from the DC voltage source is the sum of the effective power in the DC motor and the power loss (the collector loss) of the drive transistor. The power loss of the drive transistor is generally larger than the effective power in the DC motor. Accordingly, the power efficiency, defined as the percent ratio of the effective power supplied to the DC motor to the total power supplied from the DC voltage source, is rather small, for example, 10% to 30%. Particularly, in a control system for a DC motor having a wide range of outputs in rotational speed or torque), the power efficiency becomes remarkably small at low outputs (i.e. low speed or low torque).
In the prior art, there is proposed a pulse drive system for a DC motor energizing motor coils impulsively in order to improve the power efficiency. (For example, U.S. Pat. No. 4,051,417 discloses a pulse drive system for a brushless DC motor.)
Inflicting high frequency voltage pulses on the inductive coils causes the pulse drive system for a DC motor to have the following defects:
(1) Sharp reactive pulses occur at the motor coils so that a ripple of the supply current and a vibration of the motor are induced.
(2) Switching transistors may be destroyed by the reactive pulses.
(3) The pulses of voltage produce harmful electromagnetic waves in the motor coils, which induce noise in other audio apparatus.
(4) A large electromagnetic shield over the motor coils is necessary in order to equip the pulse drive system in an audio apparatus.